(AP) -- During Chris Petersen's tenure with Boise State, the Broncos have consistently been among the most productive offensive teams in college football. It's the defense, however, that can't be overlooked as the team has gotten off to yet another unbeaten start.

The 13th-ranked Broncos (6-0, 2-0) take one of the nation's top scoring defenses into Spartan Stadium on Friday night to meet San Jose State (5-2, 3-0), a matchup of the only teams in the WAC without a conference loss.

Petersen served as Boise State's offensive coordinator from 2001-05 before taking over as coach in 2006. The Broncos have ranked in the top 10 in scoring in the Football Bowl Subdivision every season since 2002, helping make up for a defense that failed to distinguish itself for much of that stretch.

That balance has shifted in 2008, however. The Broncos still have a strong offense, ranking third in the WAC with 32.5 points per game. The defense, though, has been just as responsible for the team's second 6-0 start in three seasons under Petersen.

Boise State is second in the FBS in scoring defense, allowing 10.5 points per game. It's given up more than seven points once this season, in a 37-32 road victory over then-No. 17 Oregon on Sept. 20.

"Defense has been great all year, and they just step up to the challenge each week," quarterback Kellen Moore said. "It's fun on our side to know that they are going to get out there and take care of business."

The Broncos shut down a prolific Hawaii team in a 27-7 win last Friday night, intercepting five passes, recording seven sacks and holding the Warriors to 288 yards, including a season-low 56 on the ground. The dominant defensive effort helped Boise State avenge a 39-27 loss to Hawaii in last year's conference championship game.

"It all goes back to practice and finishing every play out there," defensive end Mike T. Williams said. "Just making sure that we give our best because if we don't finish, you saw what happened out there last year."

Petersen liked what he saw from his team as he improved to 29-3 as a head coach, but still says there's room for improvement.

"We are a work in progress and we will get better," said Petersen, whose Broncos are one of nine undefeated teams in the FBS. "This game is so much about detail. It looks simple, but there are so many things that go along with making it work.

"Those kids care tremendously and they've very prideful and we'll go back to work. Everybody knows the drill. It's all about trying to get better. If we can keep that attitude, that work ethic, we'll be OK."

Petersen expects a challenge from San Jose State, which has won its first three WAC games for the first time since joining the conference for the 1996 season. The Spartans hadn't won three straight conference games at any point during a season since 2000.

Like Boise State, San Jose State has benefited from a tough defense. The Spartans have given up 16.6 points per game, second among WAC teams. They had six sacks and forced four turnovers in a 31-14 road win over New Mexico State on Saturday.

"Our defense had just an incredible night. They've been playing great all year," coach Dick Tomey said. "We had a lot of sacks, a lot of pressure, a lot of tackles for loss, and the pressure was intense. Our defensive front is a good outfit."

Tomey hopes it's good enough to end San Jose State's struggles against Boise State. The Spartans are 0-8 in the series and 0-3 under Tomey, who's in his fourth season.

San Jose State has also lost its last nine games against ranked opponents since a win over then-No. 9 TCU on Nov. 4, 2000.

The Spartans' last matchup with a Top 25 team came at Boise State on Nov. 3, a 42-7 defeat. Ian Johnson ran for two touchdowns as the Broncos outgained the Spartans 434-150.